In the preparation of conventional gas permselective composite membranes, a variety of problems are often encountered. These include, for example, low gas flux due to the thickness of the coating layer, difficulty in obtaining a thin coating layer, insufficient adhesion strength between the coating layer and the substrate, and difficulty of membrane fabrication. It is particularly difficult to prepare gas permselective composition membranes which achieve both high gas selectivity and high gas flux at the same time. In the case of asymmetric membranes, the problems include complicated fabrication procedures and limited selection of available raw materials.
In the case of composite membranes formed by plasma polymerization coating techniques, the plasma coatings are deposited in networks of highly branched and highly cross-linked segments. The interaction of the plasma polymer coating with the substrate and the unique mechanism of plasma polymer formation result in excellent adhesion of a thin deposit of coating to the substrate. Accordingly, several researchers have investigated applying techniques of plasma polymerization coating to the preparation of gas separatory composite membrane having combined high rates of permselectivity and flux.
European patent Application No. 0 134 055, by Van Der Scheer, discloses a composite dense membrane for selectively separating gases, comprising a dense ultrathin gas selective film of plasma polymerizate, a dense, highly permeable intermediate layer of conventionally-formed polymerizate and a microporous substrate supporting the plasma polymerizate film and the intermediate layer. The conventional polymer deposited directly on the substrate may be an organosiloxane or a copolymer of siloxanes and polycarbonates. The top ultrathin gas selective film is of a silicon-free plasma polymerizate. This reference specifically requires the intermediate layer, and indicates that its presence is necessary to serve two important purposes, that is, (1) to support the plasma polymerizate coating, enabling the deposition of an ultrathin top layer, and (2) to distribute the gas mixture over the microporous substrate, in order to permit the entire composite membrane to be used for gas separation. Thus, Van Der Scheer specifically indicates that high gas selectivity combined with high gas flux would not be expected from depositing a single plasma layer directly onto a microporous substrate. In direct and surprising contrast to the teachings of this patent publication, this invention has been able to prepare a composite membrane of high gas permselectivity and high flux by depositing a "soft" ultrathin plasma polymerizate layer of a specific low molecular weight organosiloxane monomer directly onto a microporous substrate by a novel plasma polymerization procedure. Additionally, this inventor has further unexpectedly discovered that organosiloxanes as a class can provide a highly permselective plasma co-polymerizate layer for direct deposition onto microporous substrates, when a "soft" organosiloxane plasma polymer is co-polymerized with a "hard" plasma polymer. The "hard" plasma polymer is obtained from an organosilane, a fluorocarbon or a hydrocarbon monomer.
In U.S. Pat. No. 4,410,338, Yamamoto, et al., gas separating membranes are disclosed wherein a microporous substrate is plasma coated with a polymer formed from an organic monomer selected from organosilanes, organosiloxanes and olefins. Yamamoto, et al., do not recognize any criticality in the molecular weight or size of the monomer used to prepare the plasma coating. Unexpectedly, the present inventor has discovered that a specific group of low molecular weight organosiloxanes can be used to prepare gas permselective membranes of increased gas selectivity combined with high rates of flux, which are not reported by Yamamoto, et al. Further, the disclosure of Yamamoto, et al., is completely silent on the use of copolymeric plasma coatings, which the present inventor has developed for gas membrane preparation. The gas membranes of Yamamoto, et al., are prepared by positioning a microporous substrate in a conventional radio frequency powered Bell Jar plasma reactor, and depositing a plasma polymerizate coating thereon. The intensity of the plasma glow zone developed in such a conventional plasma reactor is inherently weak, and the intensity decreases with the distance of the substrate from the plasma generating electrode. Thus, the polymerizate coating prepared in such a reactor cannot be intensively cross-linked, due to the weakness of the plasma glow zone intensity, and further, the deposition rate will be very low, and the composition and uniformity of the plasma polymerizate coating will vary with the position of the substrate within the reactor. This inventor has now discovered that gas permselective composite membranes can be prepared with plasma polymerizate coatings of specifically selected monomers and co-monomers, which are of a highly cross-linked structure and of an extremely uniform composition and thickness. This is accomplished by carrying out the plasma polymer deposition in an R. F. tubular plasma reactor with capacitively coupled external electrodes, wherein the plasma glow zone is controlled to the region between the electrodes and is of a higher energy intensity than has been possible with previous conventional plasma reactors.
In previous investigations reported in Thin Solid Films, 118 (1984) pp. 187-195, entitled Preparation of Gas Separation Membranes by Plasma Polymerization with Fluorocompounds, this inventor reported other less energy intensive plasma polymerization reactor systems, wherein organic monomers different than those used herein were polymerized to form coatings for gas separatory membranes. However, this inventor has now discovered that certain specific monomers and certain specific combinations of monomers and co-monomers can be used to prepare gas membranes of high selectivity and high rates of flux. Also, this invention has found that the plasma polymerization to prepare them can unexpectedly be carried out in the energy intensive plasma glow zone of a radio frequency powered tubular reactor with capacitively coupled external electrodes.
In the development of the present new gas permselective composite membranes, this inventor needed to overcome problems inherent in the use of previously available plasma polymerization techniques. As applied to the fabrication of composite membranes, conventional plasma polymerization coating methods all suffer from certain inherent disadvantages, regardless of the type of reactor systems utilized (i.e., Bell Jar reactors, A. F. tandem systems, R. F. coil-inductively coupled tubular reactors). These disadvantages, generally, are due to the fact that such conventional plasma polymerization involved deposition of the polyer onto a substrate situated in a low or uneven plasma energy density area. These disadvantages can generally be summarized as follows:
1. Non-uniformity in plasma polymer deposition rates and plasma polymer coating composition, primarily dependant on the substrate's position in the reactor.
2. Low or uncontrollable energy density levels encountered with conventional plasma reactors, whether of the Bell Jar or R. F. coil-inductively coupled tubular type, where polymer deposition takes place in the "after glow" zone, or of the A. F. type, where polymer deposition takes place in the glow zone. Low deposition rates in conventional reactors can further be attributed to the build up of plasma coating on the internal electrodes.
3. Inability to evenly and effectively coat multiple membrane substrates, due to competitive shading from the plasma glow, and due to the fact that polymer deposition rates are primarily dependant on the precise position of the substrate in the reactor.
4. Non-uniformity in coating around the exterior of the membrane substrate, for example, around the circumference of a fiber.
5. Problems in the undesirable formation of multiple chemical species, and the inability to efficiently remove waste chemical species.
6. Difficulty in controlling all of these plasma deposition parameters, particularly in scale-ups to commercial production.
In an effort to overcome these difficulties, this inventor has now developed a process, whereby plasma polymer is deposited on the microporous membrane substrate moving through the energy-intensive glow zone in the region between the external electrodes of an R. F. capacitively coupled tubular reactor. The plasma polymer deposition techniques, as carried out in this reactor system, are highly reliable, able to operate at high production rates and produce a highly desirable uniform product. It is well known in the art that microporous membrane substrates are extremely difficult to plasma coat, particular in continuous commercial productions, due to their sensitivity to manipulative stresses, such as temperature, pressure, tension, and chemical attack. However, this inventor now unexpectedly discloses that microporous membrane substrates can be plasma coated in this manner with speed and efficiency and with uniform desirable results to yield novel gas permselective composite membranes having combined properties of high permselectivity and high flux.